Industrial water systems utilizing fresh untreated lake or river water have for some years experienced infestation with an Asiatic clam, Corbicula, and other shellfish to the extent that various portions of the industrial water system become plugged by clam shells. Users of industrial water system users have traditionally utilized one of three methods for dealing with infestation of their industrial water systems by clam shells: mechanical cleaning, chemical treatment, and strainer devices.
Mechanical cleaning involves shutting down the plant, draining water from the heat exchanger, and allowing plant personnel to "rod out" the heat exchanger tubes using a rod or stiff wire. Rodding-out heat exchanger tubes is a time- and labor-intensive task. The rodding process may also damage the interior walls of the tube, leading to increased corrosion and turbulence.
Chemical treatment involves the introduction of a pesticide or other toxic substance into the water stream to control clam infestations. Such chemical treatments have at least two problems. First, chemical treatment is a continuous process, requiring the continued purchase of expensive chemicals for the life of the system. An increasingly important problem involves state and federal water quality regulations which mandate the use of monitoring or detoxification equipment on the water system's discharge to prevent the introduction of dangerous chemicals into the environment. Such equipment may add significantly to the overall cost of the chemical treatment problem.
The use of strainer devices involves the fitting of an individual strainer into or over the end of each heat exchanger tube. The strainer is designed to prevent the entry into the tube of clams, clam shells, or other debris over a predetermined size.
Examples of various different forms of strainer assemblies including some of the general structural and operational features of the instant invention are disclosed in the prior art. However, most of these previously known forms of strainer assemblies are not specifically adapted for use in conjunction with heat exchanger tubes. Another strainer device, which was designed for use with heat exchangers, is described in U.S. Pat. No. 4,590,994 to Champion. While this device represented an improvement over mechanical cleaning or chemical treatments, such devices encounter several problems including pressure drops, a tendency to clog, and cleaning difficulties.
Pressure drops in strainers are caused by a reduction of the water flow path area due to occlusion by the structure of the strainer. Another cause of pressure drops is drag and turbulence resulting from fluid flow around the structure.
A tendency to clog in strainers is caused by unnecessarily small flow passages which accumulate subcritical-sized debris and shells, i.e., of a size that would pass harmlessly through the heat exchanger tube. Another cause of clogging is strainers with structures having numerous features oriented substantially parallel to the front surface of the heat exchanger tube sheet, i.e., perpendicular to the primary direction of water flow into the tubes, thus occluding the flow path and providing opportunities for fibrous debris to accumulate. Yet another cause of clogging is strainers which protrude outward beyond the end of the heat exchanger tubes so far that they provide locations for shells and other debris to become trapped in the cavities formed between adjacent strainer devices.
Cleaning difficulties in strainers are caused by structures with large numbers of flow passages, as accumulations of shells, fibrous material, and debris often require the cleaning of individual passages. Another cause of cleaning difficulty is the accumulation of large shells and debris in the cavities between adjacent strainers.
Other prior art strainer devices, such as the "bullet style" strainer, seek to reduce pressure drops by increasing the size of the openings in the structure; however, the bullet style strainer still includes structural features which may cause turbulence and drag, clog with debris, and it extends outward from the tube sheet far enough to form debris-accumulating cavities between the adjacent strainers. In addition, these designs include ring-shaped stiffeners oriented parallel to the front surface of the tube sheet and perpendicular to the primary direction of water flow into the tubes, thus occluding the flow path and providing opportunities for fibrous debris to accumulate, also tending to clog the water inlet.
A need therefore exists for a strainer device capable of providing the necessary preclusion of unwanted objects, and yet presenting a minimum amount of pressure drop to the fluid stream. A need further exists for a strainer device which contains minimal structural components oriented perpendicularly to the primary direction of water flow into the tubes. A need also exists for a strainer device in which the strainer structure does not protrude so far beyond the end of the heat exchanger tubes as to create debris-accumulating cavities.
Yet another problem encountered in the use of prior art strainers with heat exchangers is their interference with improved leak detection procedures during maintenance. A traditional method of detecting leaks in the tubes of large heat exchangers such as those used in steam power plants is as follows: the heat exchanger is taken out of service and the water drained. Maintenance workers enter the waterbox area of the heat exchanger and remove all strainers from the heat exchanger tube inlets. This is a time-consuming process. Flexible plastic sheeting is then held against the inlet and outlet ends of the heat exchanger tubes. If any of the heat exchanger tubes has a leak, the natural vacuum present on the condenser side of the heat exchanger tubes pulls the plastic sheets tightly against the ends of the leaking heat exchanger tube. The suction of the plastic sheet against the leaking heat exchanger tube can then be visually observed by the maintenance workers, indicating which tube was leaking. An improved method of leak detection involves the use of plastic sheeting only on the outlet end of the heat exchanger, and the use of a layer of soap and water foam, which is sprayed over the inlet ends of the tubes. If a leak was present in one of the heat exchanger tubes, the foam is sucked into the end of that tube, and this can be visually detected by the maintenance personnel. Many prior art strainers must be removed before the foam spray method can be utilized, as it is often impractical to get a simple foam spray of sufficient depth to completely cover the projecting portion of the strainers. In contrast, the use of strainers which project out from the tube sheet a lesser amount than the prior art often eliminates the necessity for maintenance personnel to remove the strainers before using the foam spray method of leak detection. The less the projection of the strainer from the tube sheet, the thinner the layer of foam that is necessary, and the greater the likelihood that the layer of foam can simply be sprayed over the ends of the strainers and that the suction of the foam by leaking tubes can be observed by maintenance personnel.
Yet another problem with prior art strainers is a tendency for the protruding portion of the strainer to be broken or damaged by impact with debris in the water. Strainers which project out from the tube sheet a lesser amount than the prior art exhibit a reduced tendency to break when impacted by debris in the water stream.
One aspect of this invention provides a strainer assembly for use in conjunction with heat exchanger tubes for the purpose of preventing the entrance of Asiatic clams or other debris of predetermined size into the inlet ends of the heat transfer tubes. Another aspect of this invention provides a strainer which may be readily removed and replaced. Still another aspect of this invention provides a strainer construction which may be readily manufactured of different sizes so as to be adaptable for use in conjunction with heat transfer tubes of varying sizes. A further aspect of this invention provides a strainer assembly constructed in a manner whereby the surfaces of the strainer assembly, defining the various surface openings formed therethrough, may be readily cleaned. A final aspect of this invention provides a strainer assembly which will conform to conventional forms of manufacture, be of simple construction, and easy to use so as to provide a device that will be economically feasible, long lasting, and relatively trouble-free in operation. These, together with other aspects and advantages, which will become subsequently apparent, reside in the details of construction and operation, as more fully hereinafter described and claimed, reference being made to the accompanying drawings, forming a part hereof, where in like numerals, refer to like parts throughout.
A need exists, therefore, for provision of means by which infestation of industrial water systems by Asiatic clams be controlled, at least to a reasonable extent, and to this end, the instant invention comprises a strainer assembly which has been found to be highly effective in controlling clam infestation by clams above a predetermined size.